Great technical improvements have been achieved in the almost thirty years of cardiac pacemaker therapy. Not only has the longevity of pacemaker aggregates increased from a few months to several years, at the same time there has been a significant reduction in size. Multiprogrammable pacemakers have been made possible by modern microelectronics.
U.S. Pat. No. 4,485,818 describes a multiprogrammable pacemaker on the basis of a microprocessor. The use of lithium batteries, that are more favorable in terms of energy and have replaced the formerly customary zinc-mercury batteries with high self-discharge, has contributed to a further reduction in pacemaker size and longer longevity. Energy saving circuitry measures have reduced the pacemaker's current consumption. The development of modern electrodes that build up very small polarization voltages has also contributed to reducing the energy required for pacing.
The pacing and excitation of the cardiac muscle is subject to the laws of electrophysiology, according to which a relation is known between pulse width, which is necessary for exciting the heart and pulse amplitude. The stimulus threshold curve is an individual parameter, which results from the interference of the particular pacemaker patient's heart and the electrode. The stimulus threshold curve is not a fixed parameter, but is influenced by many different factors. These include the size, surface and nature of the electrode. The nature of the heart also plays a crucial part. In addition to these predetermined influencing factors, there are a large number of continuously changing influencing factors such as drug effects, the perfusion conditions in the heart under various working conditions, epinephrine levels in the body, diurnal variations and the type of pacing (uni- or bipolar) as well as the length of time an electrode has been implanted. These parameters have an essential influence on the individual stimulus threshold curve. Further information can be found e.g. in Alt "Schrittmachertherapie des Herzens", perimed Verlag, Erlangen, 1985, pp. 33-39 and in Ripart and Muciga "Electrode Heart Interface: Definition of the Ideal Electrode", Pace, Vol. 6, March 1983 pages 410 to 421.
Since numerous factors have a continuously changing influence on the stimulus threshold curve, the energy delivery of a pacemaker system must do justice to these fluctuations. A so-called 100% safety margin is generally set, which means that the energy delivered for pacing should be 100% higher than the minimum amount of energy necessary for exciting the heart at the time of control. The energy consumption per pacing pulse is a function of the voltage, the current and the duration of the stimulus. Assuming constant resistance, the energy consumption can also be stated as: EQU W=U.sup.2 .times.t/R (1).
In accordance with the stimulus threshold curve, a certain minimum product of voltage or current and duration of effect (pulse width) is always necessary, which is described by the familiar equation of the stimulus threshold curve: EQU U=U.sub.o (1+t.sub.c /t) (2)
U.sub.o is the voltage rheobase, the time of chronaxie t.sub.c corresponds to the pulse width at twice the rheobase voltage. Assuming a constant resistance R and combining Equations 1 and 2, one sees that the energy is lowest at the time of chronaxie t.sub.c, being EQU W(t.sub.c)=4U.sub.o.sup.2 t.sub.c /R (3)
On the other hand, a reduction of the pulse width oppositely has higher energy consumption due to a compensatory necessary increase in voltage. For example, a reduction of the pulse width to one tenth of the chronaxie requires a compensatory increase in voltage U. From Equation 2 above, the voltage U necessary in this type of pacing is eleven times U.sub.o.
This results in a total energy of EQU W=12.1U.sub.o.sup.2 t.sub.c /R.
Compared to the energy W(t.sub.c) at the optimal time t.sub.c (see Equation 3), the required amount of energy in the stated case is more than 300% the energy for pacing at the time of chronaxie. It is obvious that this energy consumption has considerable influence on the longevity of the pacemaker.
In order to obtain sufficient reliability despite the varying stimulus threshold, measures have been developed in the past for determining the stimulus threshold even after implantation.
U.S. Pat. No. 3,777,762 describes a method for non-invasive monitoring of the stimulus threshold by continuously reducing the pulse amplitude.
U.S. Pat. No. 4,305,396 describes a pacemaker which is able to distinguish between effective and ineffective pacing pulses on the basis of the after-potential variations.
The article "The Autodiagnostic Pacemaker" by A. Auerbach in PACE, Vol. 2, pp. 58 to 68, 1979, describes an autodiagnostic pacemaker which is also able to distinguish between effective and ineffective pacing pulses. A similar method is described by H. J. Thalen in the article "Evoked Response Sensing as Automatic Control of the Pacemaker Output" in G. A. Feruglio: Cardiac Pacing, Piccin Medical Books, Padua 1982, pp. 1229 to 1234.
The idea of automatically detecting the stimulus threshold does justice to the idea of safety, to prevent the pacemaker pulses from being rendered ineffective by an incorrectly low adjustment of the delivered energy and the patient from therefore undergoing asystole possibly being life threatening.
However, none of these papers indicates at what time, i.e. at which pulse width and at which pulse amplitude, a given patient is best treated with regard to energy saving operation as well as maximum safety.
The present invention is based on the problem of providing means for a cardiac pacemaker which adapts the individual energy demand necessary for safe and effective pacing to the particular possible minimum, and thus optimally utilizes the available electrical energy, thereby allowing for an even smaller pacemaker that imposes even less of a strain on the patient.
This problem is solved according to the invention in accordance with the embodiments set forth in the following specification, claims and the accompanying drawing.
The invention is thus based on the finding that if the pacemaker, or the controlling external programming system, independently measures the stimulus threshold and measurements are taken at different pulse widths and pulse amplitudes, one can determine the rheobase, which can be described as a current rheobase or as a voltage rheobase (I.sub.o or U.sub.o). As a further means to calculate the minimum energy needed for effective pacing, the internal current consumption at the pacemaker's battery can be considered as well. In conjunction with low polarizing electrodes, one can likewise determine the chronaxie with sufficient confidence by at least two measurements with different pulse widths or pulse amplitudes. Pacing with a pulse width corresponding to the chronaxie constitutes the optimal energy consumption. The repeated detection of stimulus threshold values and the chronaxie contributes to adapting the pulse delivery and thus the energy delivery to the changing needs of the patient. It makes no difference for the individual pacemaker what electrode is used to pace the heart, which means, for example, that older electrodes with poorer energy consumption are checked for optimal pacing properties just as more favorable modern electrodes are. The repeated determination of the corresponding parameters also allows for adaptation to the different fluctuations over time in the stimulus threshold curve within the individual patient. One is aware, for example, of diurnal variations of the stimulus threshold curve, drug effects, changes in the electrolyte metabolism and changes in the stimulus threshold due to intermittent cardiac ischemia. Since more than half of all pacemaker patients suffer from perfusion insufficiency in the cardiac muscle, this aspect is of major importance.
The invention can also be used in connection with an external programming device. The stimulus threshold of the particular pacemaker patient is determined by common programming and telemetry methods. A special logic in the external programming device performs measurements according to the invention and determines the most energy efficient pulse duration and pulse amplitude within an individual patient by performing automatically several, at least two, measurements of the threshold value at varying pulse widths and amplitudes. By means of constant bidirectional telemetry and transmission of the intracardiac signal from the pacemaker to the external programming unit the program unit is capable to the intracardiac signal with respect to effective and ineffective pacing impulses delivered to the heart from the implanted pacemaker, but under control of the external programming unit. In case of the first evidence of a pacing impulse to be ineffective, the energy delivered to the heart is instantaneously increased again in order to prevent the pacemaker bearer from prolonged asystolic episodes. The data is fit into the formula for the calculation of the most energy effective individual setting, above noted, with appropriate safety margin. After the external programming unit has performed this determination of the optimal energy setting for an individual heart-electrode configuration, it also takes into consideration the internal technical situation within a given pacemaker model as far as the practical technical possibilities are concerned to select a theoretically optimal energy effective setting, since those data of different models of manufacture are stored within the logic of an external programming unit. A comparison is then made of the best theoretical value with the best practical realizable value for the equipment and accordingly a pulse width and amplitude setting is selected that is practically the best as far as energy efficiency is concerned, even though this might differ slightly from the theoretically best setting. A calculation of a safety margin is also performed. This practically best setting for values of pulse width and amplitude including a safety margin is shown on a display in the programming unit. After confirmation by the supervising physician or pacemaker clinitian (nurse, or technician) this setting is transmitted to the implanted pacemaker and the pacemaker is programmed accordingly.
This way of putting the logic circuits required by this invention into an external programming unit, with telemetry communication with the pacemaker, takes into consideration the momentary situation of limited logic available within the pacemaker at present, and the energy drain on the battery in the pacemaker that would be taken if the pacemaker determines the optimal energy by itself.
Further embodiments, features and advantages of the invention may be found throughout the following description.